Titanium alloys, which are lightweight, high in specific strength, and moreover excellent in heat resistance, are used in a wide variety of fields including aircrafts, automobiles, consumer products, and the like. A typical example of the titanium alloys is α+β Ti-6Al-4V. Out of α+β titanium alloys, an alloy containing a β stabilizing element in a relatively large quantity is called a β rich α+β titanium alloy or a Near-β titanium alloy, which is widely used as a high-strength titanium alloy.
Although the definition of the β rich α+β titanium alloy or the Near-β titanium alloy is not well-defined, it is an alloy of a α+β titanium alloy that contains a β stabilizing element in a large quantity to increase the ratio of a β phase. Hereinafter, it will be referred to as a Near-β titanium alloy. Typical examples of the Near-β titanium alloy include, but not limited to, Ti-10V-2Fe-3Al, Ti-6Al-2Sn-4Zr-6Mo, Ti-5Al-5V-5Mo-3Cr, and the like. In addition, titanium alloys such as Ti-5Al-2Fe-3Mo and Ti-4.5Al-3V-2Mo-2Fe are included in Near-β titanium alloys. Mo equivalent, which is used as an index indicating a β phase stability (Mo equivalent=Mo[mass %] V[mass %]/1.5+1.25×Cr[mass %]+2.5×Fe[mass %]) is within a range of about 6 to 14 for the alloys described above.
The strength and ductility of a Near-β titanium alloy can be changed by controlling the form of the microstructure thereof through thermo-mechanical treatment. However, an excessively increased strength of a Near-β titanium alloy leads to an increased notch susceptibility, which becomes a problem in terms of practice.
Meanwhile, a titanium alloy poses a problem of a poor wear resistance when used for a sliding portion as a component for an automobile. To improve the wear resistance of a titanium alloy member, various kinds of coating and techniques such as hardened layer formation have been developed. Coating is to form a hard ceramic or a metal on a surface of a titanium alloy member by a method such as physical vapor deposition (PVD) and spraying. Coating has not come into widespread use due to its high treatment costs.
As a method inexpensive and easy to use industrially, there is a method of forming a hardened layer on a surface of a titanium alloy starting material. For example, Patent Document 1 describes a method of forming an oxide scale on a surface of a product by performing heat treatment in an atmosphere furnace. Patent Document 2 discloses a surface treatment method for a titanium-based material by which an oxygen diffusion layer is formed without generating an oxide layer by performing oxygen diffusion treatment in an oxygen-poor atmosphere.
In the case of forming an oxidized layer or an oxygen diffusion layer by causing oxygen to diffuse from the surface into the inside of a titanium alloy starting material, an oxygen concentration of an outermost layer becomes extremely high. As a result, a fatigue fracture starting from a surface occurs in a titanium alloy member, which problematically reduces fatigue strength.
Thus, there have been studied various methods for suppressing the reduction in fatigue strength or obtaining a high fatigue strength, after forming an oxidized hardened layer.
For example, Patent Document 3 proposes a method for ensuring required fatigue strength and wear resistance by performing oxidation treatment at an oxidation treatment temperature and for a time satisfying conditions. Patent Document 3 discloses that making the thickness of an oxidized hardened layer 14 μm or smaller enables the reduction in a fatigue strength due to oxidation treatment to be suppressed to 20% or less.
Patent Document 4 discloses a titanium member that is subjected to oxidation treatment and then shotpeening. In Patent Document 4, oxidation treatment is performed to set a surface hardness Hmv at 550 or higher and lower than 800, shotpeening is then performed to set the surface hardness Hmv at 600 or higher and 1000 or lower, and the thickness of an oxygen diffusion layer is set at from 10 μm to 30 μm.
Patent Document 5 discloses a technique in which a carburized layer is formed on a surface of which wear resistance or fatigue strength is required, and then an oxidized layer is formed on a portion to come in contact with other valve train components.
Patent Document 6 describes a Near-β titanium alloy that is excellent in fatigue characteristics.
Patent Document 7 describes a titanium-alloy-made engine valve on a surface of which an oxygen diffusion layer is formed. Patent Document 8 describes an engine valve made of a high-strength titanium alloy for an automobile on a surface of which an oxidized hardened layer is formed. Patent Document 9 describes a titanium alloy member that includes an outer layer made of a titanium alloy base metal including a hardened layer in which oxygen is dissolved.